Placer(redirected from Placida)
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(placer deposit), a surficial accumulation of small fragments of rocks or minerals that result from the destruction of primary deposits of useful minerals or rocks. Placers contain inclusions of valuable minerals in amounts that permit economically profitable extraction.
Placers are usually composed only of minerals that are resistant to the processes of transportation and weathering, minerals such as diamond, gold, platinum, cassiterite, monazite, ruby, sapphire, ilmenite, zircon, and amber. They are formed wherever rubble, gravel, sand, and other loose material are distributed, from place of origin to the seacoast, where the products of the destruction of the continents are ultimately carried. A number of sequential stages in the formation of placers are distinguished: formation of a placer at the point of origin (eluvial placers), on the slopes or at the foot of elevations (diluvial, solifluction, colluvial, landslide, proluvial placers), in river valleys (alluvial placers, placers of subaerial deltas and broad alluvial plains), and on the shores of seas and lakes (placers of underwater deltas, beach and shelf placers, lacustrine placers). Unrelated to these stages, glacial deposits and eolian placers occur in regions of glaciation and in arid regions. The order of formation of particular genetic types of placers is not always maintained, and sometimes the process ends in one of the early stages. Placers related to different geomorphological elements of river valleys are shown in Figure 1.
According to time of formation, placers are classified as ancient (Mesozoic, Paleozoic, and Precambrian), which are usually fossil placers; Quaternary (Anthropogenic); and modern. In addition, they are classified by mode of occurrence as surface and buried (beneath the sheets of effusive rocks, moraines, landslides, and the like). By composition they are classified as monomineralic and polymineralic; by structure, as single-layered and multilayered; and by shape, as mantle, sheet, lens, ribbon, or pocket. Placers may be several hundred meters and even several kilometers long, from a few meters to several tens of meters wide, and from a few centimeters to tens of meters thick. Proceeding from top to bottom, the following basic parts are distinguished in the structure of the most common types of placers: peat, usually with few, if any, valuable rock components; sand, productive beds containing valuable components in industrial quantities; and bedrock, the surface on which the placer is located.
The best-known placers are the alluvial gold-bearing placers in the Kolyma and Chukchi regions of the USSR and in Alaska and California; the diamond placers in the Republic of South Africa and in Namibia; the beach placers of cassiterite in Malaysia, Indonesia, Thailand, and Burma; and the placers of titanium minerals in the Ukraine, New South Wales (Australia), India, and Florida.
Placers produce roughly 50 percent of the world total of diamonds, titanium, gold, and tin, about 10 percent of the world total of platinum, and 100 percent of the world total of titanium minerals and amber.
REFERENCESBilibin, Iu. A. Osnovy geologii rossypei, 3rd ed. Moscow, 1956.
Osnovypoiskov rossypei. Leningrad, 1961.
Trofimov, V. S. “Stadii obrazovaniia rossypei i ikh fatsii.” In the collection Voprosy geologii Kol’skogo poluostrova. Moscow-Leningrad, 1962.
Geologiia rossypei: Sb. st. Moscow, 1965.
V. S. TROFIMOV
Working. Placers are worked by the underground and open-cut methods. The underground method is usually used to work permafrost placers more than 10–15 m deep and unfrozen placers more than 15–40 m deep.
The most common method of working placers is mechanized opencut mining, which makes it possible to successfully work placers 15–18 m thick—up to 60 m thick if dredges are used. Rock containing sand is fed to gravitational concentrating devices, where the rock is washed in a stream of water to extract gold, platinum, and crystals (diamonds, optical quartz) and concentrates of nonferrous metals (tin, tungsten, tantalum).
Placer working involves preliminary, opening, preparatory, and extraction work. Preliminary work entails drying or flooding the placer region, in the latter case making use of amphibious mining machines. Opening work creates access to the placer horizons, sets extraction machines on the deposit, and provides transportation links. In preparatory work, the surface is cleaned, the peat is removed, the operating blocks are cut, and seasonally frozen and permafrost ground are thawed. Extraction work involves excavating and the washing out of sand. The technology of extraction work is determined primarily by the method of excavating productive rocks and the system used for working, that is, the order and procedure followed in extraction and cutting.
In the USSR, opencut placer mining makes use of multi-bucket dredges that hold up to 600 liters per bucket, bulldozers tractor-mounted with power ratings up to 283 kilowatts (385 hp), power-driven wheeled scrapers with scoops that hold up to 34 cu m, and powerful dragline excavators.
REFERENCESLeshkov, V. G. Sovremennaia tekhnika i tekhnologiia drazhnykh rabot. Moscow, 1971.
Shorokhov, S. M. Tekhnologiia i kompleksnaia mekhanizatsiia razrabotki rossypnykh mestorozhdenii, 2nd ed. Moscow, 1973.
Sulin, G. A. Tekhnika i tekhnologiia razrabotki rossypei otkrytym sposobom. Moscow, 1974.
V. G. LESHKOV